This invention relates to current fed inverters, and more particularly to a thyristor current fed inverter with commutation independent of load inductance having a variety of applications.
In some circuits with current fed inverters, which are also known as controlled-current inverters, load capacitors are used to obtain a leading power factor and provide a source of commutating voltage for the inverter SCR's. Viewed in another way, the tank circuit formed by the paralleled load and load capacitors permits natural commutation of the inverter. This type of inverter requires relatively large load capacitors to maintain commutation. A circuit that realizes a forced commutation of an inverter built with mercury are rectifiers is described in the book "High Voltage Direct Current Power Transmission" by C. Adamson and N. G. Hingorani, Garraway Ltd., London, England, copyright 1960, pages 121-123. In addition to the normal three phase bridge, this HVDC inverter further consists of a pair of auxiliary valves and a commutating capacitor located between the midpoint of the auxiliary valves and the neutral of a wye-connected output transformer winding. One of the auxiliary valves is fired to turn off a conducting main valve. However, since the commutating capacitor is connected to the load midpoint, the inductance of the load influences the commutation time and capacitor voltage magnitude. Also, the commutating voltage spikes are applied to the load. The principal object of the present invention is to attain reliable commutation in a current fed inverter which is independent of the valve of load inductance and relatively simple to implement.